1. Field of the Invention
This invention relates to a device for implantation in the human body that acts as a conductor of electricity for stimulating tissue and for sensing bio-electric activity emanating from the tissue. This invention especially relates to artificial cardiac pacemakers, but may be utilized with other devices requiring electrical conductors and stimulating electrodes within the body such as artificial implantable defibrillators, neuro stimulators, muscle stimulators, cochlear implants, and other devices.
2. Description of the Related Art
Cardiac pacemakers typically consist of two major portions. The first portion is the pulse generator which consists of a power source and electronics associated with producing electrical pulses, and the second portion consists of electrode leads for applying the generated pulses to body tissues. Conventional electrode leads typically consist of three primary parts. They include a lead body which provides a path for electrical conduction from the pulse generator to the body tissues, a proximal connector which connects the lead body to the pulse generator, and a distal electrode which delivers the electrical impulses to the body tissue. Common electrode leads are either unipolar (one electrode) or bipolar (two electrodes). Multipolar leads (more than two electrodes) are needed for use with chronic heart patients, and a number of conductors may also be desired for sensory inputs to measure such factors as oxygen concentration, temperature, blood flow or pressure and motion detection. Such multiple conductor leads are impractical with current technology however, because the many conductors needed cause the electrode lead diameter to become excessive. Similarly, the multiple leads required for sensor inputs are not practical under current technology because the electrode lead diameter becomes excessive.
Further problems with multiple electrode leads are caused by the bulky connector constructions that are conventionally used. Conductors are typically connected by mechanical means which require additional components such as crimping support pins or tubes. These connectors add to the size and weight of the implants and reduce lead flexibility, particularly as the number of conductors increases.
Currently used implantable leads consist of an electrical conductor with a layer of electrical insulation. The conductor may be in the form of a straight metal wire or a helical coil. Solid wires, however, are prone to fracture from fatigue caused by bending stresses associated with body implants. Metal coils have higher fatigue resistance, but their bulkier construction reduces design flexibility. Conductors made of platinum and stainless steel were originally used for such applications, but their high cost or relatively poor corrosion resistance led to the development of more fatigue-resistant alloys and composites, including those sold under the trademarks "MP35N" and "ELGILOY," and silver/stainless steel composite materials. More recent developments have included multifilar tensile wires, and designs incorporating carbon (graphite) fiber or tungsten as a conductor have been proposed. Materials for insulating these conductors include silicone rubber, polyurethane, and other insulating materials, such as that sold under the trademark "TEFLON."
There have been several attempts at integrating the conductor and insulator by using conductor wires that are wrapped helically around an insulator core and then encapsulated by a second insulating layer. These are disclosed in U.S. Pat. No. 3,760,812 issued to Timm et al., U.S. Pat. Nos. 3,485,234, 3,585,707 issued to Stevens, and International patent application No. PCT/U583/00827 to Berkley. However, these integral metal electrode leads suffer from many of the aforementioned drawbacks that are associated with all metal conductor/polymer insulator systems.
The use of metallic conductors with a separate insulator and mechanical means for connecting the conductors to electrodes or pulse generating mechanisms have a number of disadvantages. The following problems are associated with such devices:
1. It is difficult and expensive (i.e., labor intensive) to terminate electrode leads of conventional design due to the fact that the alloys utilized cannot be readily welded or bonded. Also, the helically coiled conductors are expensive to wind and they require precision equipment and a great deal of process control during manufacture.
2. Metals generally are relatively dense compared with the density of body tissues and thus cause mechanical irritation to body tissues.
3. Although metals presently used in electrode leads are selected for their high corrosion resistance, the possibility of corrosion cannot be entirely eliminated when there are metal-to-metal connections.
4. Metals generally have a finite fatigue life so that designing electrode leads for long-term performance often limits design possibilities because of reduced conductivity and increased size.
5. Design flexibility with currently used electrode leads is further limited by the sizable helix diameter required for fatigue resistance and by the insulation thickness dictated by safety requirements.
6. With present technology, it is impractical to design an electrode lead in which a distal stimulating tip and a proximal end connector are an integral part of the design.
7. Multipolar conductors according to present technology require excessively large leads that have significantly reduced flexibility.